ANALOGS OF RIBOFLAVIN AND FAD 559 



phorylated or in the form of a dinucleotide analog, and such has not yet 

 been found. 



Many flavin-dependent enzymes bind FAD or other flavin coenzymes very 

 tightly and it is difficult to understand how quinacrine could displace these, 

 or how exogenous FMN or FAD could antagonize the action of quinacrine 

 if this is the case. If an enzyme after extraction is flavin-dependent and is 

 catalytically active, it must have very tightly bound coenzyme; perhaps 

 quinacrine can react with the bound coenzyme but there is no evidence for 

 this. Certainly the failure of quinacrine to inhibit should not be taken as 

 evidence for the absence of a flavin component. Actually it must be said 

 that many of the experiments with quinacrine have not been done properly. 

 In some instances one concentration of quinacrine has been used and, if 

 inhibition of any degree is noted it is stated that this is evidence for a 

 flavoenzyme, even though no antagonism has been demonstrated; it should 

 be obvious that no conclusions can be drawn from these results. In other 

 cases two experiments have been run, one with quinacrine alone and one 

 with both quinacrine and either FMN or FAD; if the inhibition is less in 

 the presence of the FMN or FAD it is concluded that quinacrine is inter- 

 fering with the normal function of a flavin coenzyme. A control with FMN 

 or FAD alone must also be run, since in many cases these substances will 

 stimulate activity. The results of Bargoni (1963) are difficult to interpret 

 in that FAD would prevent the inhibition by quinacrine only if the FAD 

 were incubated with the enzyme for 30 min before the inhibitor is added. 

 The binding of quinacrine to some enzymes is readily reversible, but yeast 

 lactate dehydrogenase is irreversibly inactivated; FAD will slow this inac- 

 tivation but will not reactivate (Iwatsubo and Labeyrie, 1962). Several 

 suggestions as to the design of such antagonism tests may be made: (1) use 

 a flavin derivative which is the most likely coenzyme involved, (2) use co- 

 enzyme-dissociated and reconstituted enzymes whenever possible, (3) al- 

 ways have a control with the reversor alone, and (4) attempt to establish 

 competitive relations between the quinacrine and the reversor. 



Effects of Quinacrine on Metabolism 



Quinacrine and other antimalarials were found by Fulton and Christo- 

 phers (1938) to depress the respiration of trypanosomes. Concentrations as 

 low at 0.004 mM exert inhibitory effects on multiplication but it requires 

 0.3 mM to inhibit the respiration 11.5%, at which concentration the count 

 is reduced 26.3%. These observations and several others afterward dem- 

 onstrate that concentrations presumably higher than are present in vivo 

 must be used to depress the respiration of these organisms. Wright and 

 Sabine (1944) studied the effects of quinacrine on the respiration of rat 

 tissue slices. In most instances there is an initial stimulation followed by a 

 slowly developing inhibition, often not complete after 100-200 min. Liver 



